Quadrature amplitude modulation (QAM) of transmission carriers by digital signals is a known method of digital modulation. For two phase state modulation, each bit has two reference phases, one phase for a value corresponding to one and one phase for a value corresponding to zero. The phases are shifted by 180.degree. from each other.
In a four-phase QAM (or quadrature-phase-shift-keyer, QPSK) a carrier wave takes each of four different phase values (90.degree. separated) depending on the values assumed by two bits. Such a four phase QAM (QPSK) is shown in FIG. 1 and includes a quadrature phase shift keying (QPSK) circuit 10. A pair of digital inputs D1 and D2 are applied to mixers 12 and 14, respectively. The output of a local oscillator 16 is applied to a 90.degree. hybrid 18 where it is separated into sine and cosine values that are respectively applied to mixers 12 and 14. The outputs from mixers 12 and 14 are summed in a summing circuit 20 to provide an output on line 22.
While inputs D1 and D2 take the form of binary values 1 and 0 within QPSK 10, those values are converted by level shifters (not shown) to d1 and d2, which exhibit digital values of +1 and -1, respectively. In this manner, a quadrature phase output from summing circuit 20 appears in the form of d1Asin.omega.t+d2Acos.omega.t.
In FIG. 2, the phases emanating from summing circuit 20 are shown by an I/Q plot of phase vectors 24, 26, 28 and 30. If values d1 and d2 are +1 and -1 respectively, the output signal on line 22 lags the phase of local oscillator 16 by 45.degree.. Similarly, if d1 and d2 are, respectively, -1 and +1, the output phase lags by 135.degree., etc. QPSK 10 is thus enabled to output four separate phase signals in dependence upon the values of inputs D1 and D2.
In FIG. 3, a QAM is shown that enables the generation of 16 separate phase and amplitude signals in accordance with four digital inputs D1-D4. In this instance, a pair of QPSK modulators 30 and 32 are employed, each operating identically to QPSK 10 of FIG. 1. In this case, however, the output of QPSK 32 is fed through a 6 dB attenuator 34 before being applied to summing circuit 36. The resulting output on line 38 is a vector addition of the outputs of QPSK modulators 30 and 32 and is shown in FIG. 4.
As an example, if the output from QPSK modulator 30 exhibits the phase shown by vector 40, and the output from QPSK 32 is a signal having a phase as indicated by vector 42, then the resulting signal on output line 38 exhibits a phase as shown by vector 43 to a point 44. The 16 states of the four digital inputs D1-D4 cause the generation of a "constellation" of 16 points. A vector drawn to each point illustrates the phase and amplitude of a resultant output signal in response to the indicated digital input values.
Various modifications of the above noted prior art circuits appear in the following patents. In U.S. Pat. No. 4,571,549 to Lods et al., a 16 QAM is shown that converts a train of binary data signals into 16 predetermined phase and amplitude values. In U.S. Pat. No. 4,464,767 to Bremer, multiple, synchronous, QAM transmitters are employed that respond to an input binary bit pattern to produce output signals having phase and amplitude states defined by a 64 point constellation.
U.S. Pat. Nos. 4,168,397 to Bradley and 4,804,931 to Hulick show further versions of QAM devices for producing multi-state outputs in accordance with digital signals. Bradley describes an eight-phase PSK modulator, whereas Hulick describes a multiphase quadrature system that employs a plurality of cascaded combining circuits to achieve the multiphase output. U.S. Pat. No. 4,039,961 to Ishio describes a demodulator for a 16 QAM signal wherein the reference carrier is extracted from the received signal and is regenerated for demodulation purposes.
In all of the above noted prior art, the described modulators provide regularly arrayed phase, amplitude outputs in accordance with determined digital inputs. In each instance, the phase output is predetermined by the digital input value.
It is an object of this invention to provide a QAM system that is adaptive and can be adjusted in accordance with transmission system element characteristics.
It is another object of this invention to provide a digital modulation system that is adaptive and enables adjustment of the converter's output in accordance with predetermined control inputs.
It is a further object of this invention to provide a digital modulation system having a number of output amplitude/phase states that greatly exceed the number of possible digital transmission states, thereby enabling amplitude/phase states to be selectively chosen to compensate for non-linear and/or time dispersive elements of a transmission system.